1. Technical Field
The present invention relates to an oven controlled type crystal oscillator for surface mounting (hereafter “oven controlled oscillator”), in particular to an oven controlled oscillator which facilitates a low profile.
2. Background Art
Oven controlled oscillators, being able to impart a constant operating temperature to crystal resonators in particular, can eliminate frequency changes that result from frequency temperature characteristics, thereby stabilizing the oscillation frequency. Thus, stability of the oscillation frequency is obtained in the order of parts per billion, for example, for such applications as a frequency source in a base station. In recent years, although such highly stable oven controlled crystal oscillators have been reduced in size and adapted for surface mounting, demand exists for further reductions in height.
3. Prior Art
FIG. 11A, 11B, and FIG. 12 illustrate a conventional example of an oven controlled oscillator, wherein FIG. 11A is a schematic circuit diagram of the oven controlled oscillator, FIG. 11B is a cross-sectional view of the construction thereof, and FIG. 12 is a cross-sectional view of a crystal resonator.
The conventional oven controlled oscillator, as shown in FIG. 11A, comprises an oscillation circuit 3 having a crystal resonator 1 and an oscillation section 2, and a temperature control circuit 4 which imparts a constant operating temperature to the crystal resonator 1 in particular. The oscillation circuit 3 is, for example, a Colpitts type, subject to voltage control by the connection of a voltage variable capacitative element 5Cv to the crystal resonator 1. The oscillation circuit 3, in addition to an oscillation stage (oscillation section), also comprises a buffer stage, for example.
The crystal resonator 1, as shown in FIG. 12, for example, accommodates a crystal blank 7 within a chamber body 6 having a concave shape in cross-section. The crystal blank 7, as shown in FIG. 13, has excitation electrodes 7a on both principal surfaces thereof, and, for example, lead-out electrodes 7b extend to both sides of one end thereof. As shown in FIG. 12, the both sides of one end from which the lead-out electrodes 7b extend are fixed to the inside bottom surface of the chamber body 6 by an electrically conductive adhesive 8. Furthermore, a metal cover 9 is joined by seam welding or the like to a metal ring 11 provided at the open end surface of the chamber body 6, so as to hermetically seal the crystal blank 7 within the chamber body 6. On the outside bottom surface of the chamber body 6, external terminals 10 are provided of which two are electrically connected to each of the excitation electrodes 7a and lead-out electrodes 7b of the crystal blank 7, and the metal cover 9.
The temperature control circuit 4, as shown in FIG. 11A, comprises an op-amp 50A and a power transistor 5tr. The op-amp 50A compares a reference voltage derived from divided voltage resistors 5 (R1, R2) against a temperature detection voltage derived from a divided voltage resistor 5R3 with a thermal sensor, for example a thermistor 5th, at one input, and outputs a control voltage. The power transistor 5Tr increases or decreases the collector current based on the control voltage, thereby controlling the heat generated by a chip resistor (hereafter “heating resistor”) 5h serving as the heat-generating resistor. As a result, in particular, the operating temperature of the crystal resonator 1 is controlled constant.
In such an oven controlled oscillator, as shown in FIG. 11B, for example, a first substrate 11a and a second substrate 11b are installed on a surface mounting base 13 by metal pins P, and a metal cover 9 is bonded thereto. On each of the first and second substrates 11a and 11b, circuit patterns are formed which are not shown in the figure. The first substrate 11a is made of a flat piece of ceramic, and the second substrate 11b is made of a glass epoxy resin with an opening section 12 at its center and has larger external dimensions in plan view than the first substrate 11a. Moreover, in the surface mounting base 13, at least the surface mounted oscillator 1 is disposed on the opposing bottom surface side of the first substrate 11a, and on the top surface side thereof are disposed, for example, the heating resistor 5h, the thermal sensor 5th such as a thermistor, and the power transistor 5tr. Other circuit elements 5 associated with the oscillation circuit 3 and the temperature control circuit 4 are disposed in the outer peripheral sections of the top and bottom surfaces of the first substrate 11a. 
On the bottom surface side of the first substrate 11a, so as to face the second substrate 11b, the circuit elements 5 including the crystal resonator 1 on the bottom surface side are inserted into the opening section 12 of the second substrate 11b. Furthermore, the outer periphery of the first substrate 11a is joined to the peripheral surface of the opening section 12 of the second substrate 11b. For example, the ends of the circuit pattern which extend to the four outer peripheral corners of the first substrate 11a and the corresponding one end of the circuit pattern of the second substrate 11b are electrically joined by solder or the like. Furthermore, the other end of the circuit pattern of the second substrate 11b extends to the small openings at the four corners where the metal pins P are inserted.
The surface mounting base 13 is, for example, made of a glass epoxy resin in a dual layer of substrates 13a and 13b. Furthermore, external terminals 14 serving as mounting terminals extending from electrode pads at the lamination plane via the outer surface, are provided at the four corners of the outside bottom surface of the surface mounting base 13. At the four corners of the surface mounting base, metal pins P are provided which are joined to the electrode pads by solder (not shown). The metal pins P are inserted into the small openings at the four corners of the second substrate 11b, and are electrically connected to the other end of the circuit pattern by solder while holding the second substrate 11b in place.
By this construction, particularly because the first substrate 11a is ceramic having excellent thermal conductivity, and the heating resistor 5h and the crystal resonator 1 are disposed thereon, thermal conductivity to the crystal resonator 1 is enhanced. Furthermore, the outer periphery of the first substrate 11a faces and electrically connects to the second substrate 11b made of a glass epoxy resin which has poor thermal conductivity, and the outer periphery of the second substrate 11b is held in place by metal pins P connected to the external terminals 14. Accordingly, because the heat produced by the first substrate 11a (made of ceramic) is shielded by the second substrate 11b (made of epoxy resin), thermal efficiency is enhanced. Moreover, because the first substrate 11a and the second substrate 11b, as in the prior art (patent document 2), do not need to adopt a two-tiered construction using metal pins P, the height of the oven controlled oscillator can be reduced.